As disclosed in JP, U, 57-174562, for example, an offset boom type construction machine is arranged such that a lower boom is attached to a body of the construction machine for a vertical pivotal movement, an upper boom is attached to a fore end of the lower boom for a lateral pivotal movement, and a cylinder stay is attached to a fore end of the upper boom for a lateral pivotal movement. The construction machine also comprises an offset boom which is capable of offsetting upon a lateral swing of the upper boom, an arm attached to the offset boom via the cylinder stay for a vertical pivotal movement, and a working attachment such as a bucket attached to a fore end of the arm. A cab is provided on the machine body laterally of the lower boom.
In that offset boom type construction machine, when the arm is folded while the offset boom is offset toward the cab side, or when the offset boom is made offset toward the cab side while the arm is maximally folded, there is a risk that the bucket may interfere with the cab. A safety device is, therefore, required to prevent the interference between the bucket and the cab.
Conventional safety devices for that purpose are divided into mechanical ones and electro-hydraulic ones. As disclosed in JP, U, 2-37953, one of the known mechanical safety devices comprises a stopper in the form of a long plate having an elongate hole bored in one end and a pin hole bored in the other end, the stopper being attached to and along an offset cylinder by the use of joint pins at both ends of the offset cylinder. With this structure, when the offset cylinder is extended and contracted, the upper boom is swung laterally and, simultaneously, the joint pin is moved along the elongate hole. The sizes of the stopper and the elongate hole are selected to prevent the upper boom from approaching the cab side beyond a certain angle, whereby the bucket is kept from entering a risky range of possible interference with the cab.
However, since the above prior art is constructed to impede a lateral swing of the upper boom under a restriction imposed by the stopper, the upper boom can be in no way swung to the same side as the cab. In an attempt to perform work while offsetting the offset boom to the same side as the cab, the stopper must be removed before starting the work. The removal of the stopper means disassembly of the safety device; hence the safety device can no longer operate. Accordingly, the work is quite dangerous.
It is an object of the present invention to provide an offset boom type construction machine with which an offset boom can be offset toward the cab side and a working attachment can be prevented from interfering with a cab.